Tuberculosis, one of the great scourges of humankind, is the leading cause of death worldwide from a single infectious disease. Although the incidence of TB has again begun to decline in the U.S., TB remains a significant health problem in this country, most frequently affecting the elderly, the homeless, individuals with AIDS and immigrants from nations where TB is endemic. Although the causative agent of TB, Mycobacterium tuberculosis, was identified a century ago, knowledge about fundamental physiological capabilities, the genetics and the mechanisms of pathogenicity of M. tuberculosis is only now beginning to emerge. We have identified several genes which are expressed by M. tuberculosis H37Rv when the bacilli are growing in human macrophages in culture, but which are not expressed by the bacilli when they are growing in laboratory broth culture. We hypothesize that these genes and their products may be important in the survival and growth of M. tuberculosis in macrophages and may contribute to the pathogenicity of the tubercle bacilli. We propose to (1) determine the contribution of specific macrophage-expressed gene products to survival and growth of M. tuberculosis in macrophages by molecular and genetic characterization of genes on a cosmid that appear to be coordinately expressed, a putative response regulator gene, and mceD, a gene that has been shown to enhance survival of E. coli in cultured macrophages; and (2) to identify and characterize genes and gene products of M. tuberculosis that are expressed at early times after phagocytosis, at late times, and throughout growth in human macrophages in culture.